[Paper Review] Localization and interaction of indirect excitons in GaAs coupled quantum wells

Alexander A. High, Aaron T. Hammack+5 more|arXiv (Cornell University)|Apr 30, 2008

Semiconductor Quantum Structures and Devices28 references18 citations

TL;DR

This study introduces an elevated in-plane trap technique in GaAs coupled quantum wells to achieve effective cooling of indirect excitons, enabling observation of sharp photoluminescence lines from individual localized exciton states in a disorder potential. The key finding is that exciton-exciton interactions cause homogeneous line broadening that dominates at high densities, with localized states showing minimal broadening due to a Coulomb blockade-like effect limiting one exciton per potential minimum.

ABSTRACT

We introduced an elevated trap technique and exploited it for lowering the effective temperature of indirect excitons. We observed narrow photoluminescence lines which correspond to the emission of individual states of indirect excitons in a disorder potential. We studied the effect of exciton-exciton interaction on the localized and delocalized exciton states and found that the homogeneous line broadening increases with density and dominates the linewidth at high densities.

Motivation & Objective

• To investigate the localization and interaction effects of indirect excitons in a disordered potential within GaAs coupled quantum wells.
• To overcome limitations of conventional traps by introducing an elevated in-plane trap to reduce effective temperature and enhance resolution of individual exciton states.
• To study the role of dipole-dipole interactions in modifying the linewidth and energy levels of localized versus delocalized exciton states.
• To distinguish between homogeneous and inhomogeneous broadening mechanisms in exciton emission spectra under varying excitation densities.
• To determine the maximum occupancy of localized potential minima and assess the impact of screening on disorder potential.

Proposed method

• An in-plane trap is created using a patterned electrode structure (source, gate, and trap electrodes) to generate a spatially modulated potential via applied voltages.
• Elevated traps are formed by applying lower voltage to the central trap electrode compared to surrounding gates, raising the exciton energy in the trap relative to the surroundings.
• Photoluminescence (PL) spectroscopy with 0.18 meV resolution is used to measure emission spectra, with spatial resolution of 2 μm and laser excitation focused to ~5 μm.
• Theoretical modeling employs equations for homogeneous broadening (Γ_hom) due to exciton-exciton scattering, exciton-phonon scattering, and recombination lifetime, with key parameters derived from the dipole-dipole interaction potential.
• Inhomogeneous broadening is modeled as a function of disorder potential amplitude, with screening effects included via density-dependent reduction of the effective disorder potential.
• Theoretical linewidths are calculated using integrals over momentum and energy space, incorporating temperature-dependent factors and scattering matrix elements for inelastic and elastic processes.

Experimental results

Research questions

• RQ1 How does the elevated trap technique improve the resolution of individual exciton states in a disordered potential?
• RQ2 What is the role of exciton-exciton interactions in determining the linewidth of localized and delocalized exciton states?
• RQ3 To what extent does screening of the long-range disorder potential reduce inhomogeneous broadening at high exciton densities?
• RQ4 Can the occupancy of individual potential minima be limited by many-body interactions, and how is this reflected in spectral line shapes?
• RQ5 How do the relative contributions of homogeneous and inhomogeneous broadening evolve with increasing exciton density?

Key findings

• Sharp photoluminescence lines with full width at half maximum (FWHM) as low as 0.18 meV were observed in the elevated trap regime, indicating resolution of individual localized exciton states.
• In the normal trap, emission spectra were structureless with FWHM >1 meV, indicating strong inhomogeneous broadening due to disorder, while the elevated trap suppressed this effect.
• Homogeneous broadening increased with exciton density and dominated the linewidth at high densities, with Γ_hom ≈ ħ/τ_x-x derived from a many-body scattering model.
• The localized state at line 2 showed saturation in intensity ratio with increasing energy difference, indicating a maximum occupancy of one exciton per potential minimum due to dipole-dipole repulsion.
• The estimated exciton localization length (l_loc ~ 10 nm) is smaller than the exciton Bohr radius, supporting the Coulomb blockade-like behavior in localized states.
• Theoretical modeling confirmed that homogeneous broadening of localized states is smaller than that of delocalized states due to an energy gap ε that suppresses available final states for scattering.